Caitlin takes Barry out to a local karaoke bar to help them both find new loves. Caitlin strikes out but Barry meets a sports reporter and asks her out for a date. Meanwhile, Cisco considers Hartley's dangerous offer; and the team search for Shawna, a meta-human with teleportation powers who just busted her boyfriend out of Iron Heights.

While Barry, Harrison, and Caitlin hunt down Ronnie, who has attacked a man, Joe recruits Cisco to help him learn the truth about who murdered Nora Allen.

Martin and Ronnie are separated, only to realize that they must learn how to rejoin before Eiling hunts them down and kills them... and the Flash. Meanwhile, Joe shows Barry the 3D images from the Nora Allen crime scene, and Barry asks Martin for advice on time travel.

Wells presents Barry with a life-changing choice. Dr. Martin Stein and Ronnie Raymond return to help the S.T.A.R. Labs team with this final fight.

Picking up months after the Singularity attacked Central City, Barry is still struggling to forgive himself for Eddie’s death. Concerned about putting his friends in danger, Barry has pushed everyone away and has chosen to protect the city on his own. When a meta-human named Atom Smasher attacks the city, Iris tells Barry that he needs to let his friends help him protect the citizens of Central City. Meanwhile, Cisco helps Joe with his Meta Task Force.

Jay Garrick, a mysterious man from Earth-2, appears at S.T.A.R. Labs with a dire warning about an evil speedster named Zoom, who is set on destroying The Flash. Barry and the team must decide if they can trust this stranger even as they face yet another powerful meta-human. Meanwhile, Joe must deal with a determined officer named Patty Spivot who wants to join his meta-human task force.

When Barry and the team find out Captain Cold has been kidnapped, they form an uneasy alliance with his sister, Lisa. However, Barry feels double-crossed when he finds out Snart is working with his father, Lewis Snart on a heist. Meanwhile, Joe is faced with a difficult decision.

Barry and the team look for another Firestorm match for Dr. Stein. When the team meets Jefferson "Jax" Jackson, Caitlin has her reservations about whether Jax is the right match for Dr. Stein. Iris surprises Joe while Barry and Patty grow closer.

When aliens called the Dominators attack Central City, Barry speeds over to Star City to ask the Green Arrow for help in stopping them. Realizing the battle against the Dominators is bigger than they thought, they track down the Legends of Tomorrow, and Barry and Cisco create a breach to bring back a secret weapon – Supergirl. Once all of the super friends are assembled, they come up with a plan to attack but a shocking secret related to Flashpoint is revealed and suddenly no one is sure who they can trust. Meanwhile, Wally wants to join the battle against the aliens but Iris tells him he’s not ready.

The crossover starts on Supergirl S02E08 Medusa (I). It continues on Arrow S05E08 Invasion! (III) and DC's Legends of Tomorrow S02E07 Invasion! (IV).

Barry and Iris's wedding brings the gang together, but things go awry when villains from Earth-X attack the ceremony. All of the superheroes band together with help from their super friends like Citizen Cold, The Ray, Felicity Smoak, Iris West and Alex Danvers to take on their most formidable villains yet. Earth's mightiest heroes - Green Arrow, Supergirl, The Flash and White Canary - lead their teams into battle to save the world.

The crossover starts on Supergirl S03E08 Crisis on Earth X (I) and Arrow S06E08 Crisis on Earth X (II). It continues on DC's Legends of Tomorrow S03E08 Crisis on Earth X (IV).